1. Field of the Invention
This invention relates to an optical information processing apparatus for effecting recording and or reproduction of information by scanning the information tracks of an optical recording medium by at least one condensed light beam while effecting tracking and/or focusing control.
2. Related Background Art
In recent years, commercialization and development of optical information processing apparatuses such as electronic file systems utilizing compact discs or postscript type discs and optical disc systems using erasable magneto-optical materials or phase shift type materials have been flourishing.
Also, attention has recently been paid particularly to an optical card system which effects the recording and reproduction of information on an optical recording medium in the form of a card (hereinafter referred to as an optical card). The optical card is characterized by its ease of portability resulting from its form, and its greater information storage capacity per area than a disc.
Recording is effected on the above-described recording medium by such that a light beam modulating in accordance with recording information. The light beam is converged into a minute spot and is scanned on the surface of the medium, whereby information is recorded as optically detectable record pit rows (information tracks). In such case, in order to record the information accurately without any problems such as intersection between the information tracks, it is necessary to control the applied position of the light beam in a direction perpendicular to the scanning direction (tracking control). It is also necessary to control the applied position of the light beam in a direction perpendicular to the surface of the optical card (focusing control) in order to apply the light spot as a minute spot of stable size in spite of any bending or mechanical error of the optical card. Tracking and focusing controls are also necessary during reproduction.
FIG. 1 of the accompanying drawings is a schematic view showing an example of the construction of an optical information processing apparatus provided with tracking and focusing control means. Such an apparatus is described in detail, for example, in Japanese Laid-Open Patent Application No. 62-239333 (corresponding U.S. Application Ser. No. 33,789, which was abandoned in favor of U.S. Application Ser. No. 479,205).
In FIG. 1, a light beam emitted from a light source 1 such as a semiconductor laser is collimated by a collimator lens 2 and divided into three beams by a diffraction grating 3. These divided light beams are reflected by a beam splitter 4, are condensed on an optical card 6 by an objective lens 5 and forms three beam spots S.sub.1, S.sub.2 and S.sub.3. The light beams reflected by the optical card 6 again pass through the objective lens 5 and are transmitted through the beam splitter 4, whereby they are separated from the incident beams. These reflected beams are reflected by a mirror 7, are condensed by a sensor lens 8 and a cylindrical lens 9 and enter optical sensors 10.sub.1, 10.sub.2 and 10.sub.3. These optical sensors 10.sub.1, 10.sub.2 and 10.sub.3 are disposed so as to receive the light beams from the beam spots S.sub.1, S.sub.2 and S.sub.3, respectively.
The light receiving surface of the optical sensor 10.sub.2 is divided into four sections as shown in FIG. 2 of the accompanying drawings, and the sum of the detection signals of the two sets of opposed light receiving surfaces is differentiated, whereby astigmatism introduced by the aforementioned cylindrical lens 9 is detected and a focusing signal is obtained by the principle of the known astigmatism method described in U.S. Pat. No. 4,023,033, for example, also, when the information is to be reproduced, a reproduction signal is obtained from the optical sensor 10.sub.2. The detection signals of the optical sensors 10.sub.1 and 10.sub.3 are differentiated by a differential amplifier 11, and output as a tracking signal S.sub.T from a terminal 12. This tracking signal S.sub.T and the aforementioned focusing signal are fed back to a lens actuator 18 by a circuit, not shown, and the objective lens 5 is moved in a direction perpendicular to the optic axis and in the direction of the optic axis, whereby tracking and focusing controls are accomplished.
The optical card 6 is reciprocally moved in the direction of arrow R by a driving mechanism, not shown, whereby the light spots S.sub.1, S.sub.2 and S.sub.3 scan on the optical card. Also, an optical head 19, including the aforedescribed optical system, is designed so as to be movable in a direction perpendicular to the direction of arrow R, i.e., a direction perpendicular to the plane of the drawing sheet in FIG. 1, for the purpose of track access.
FIG. 3 of the accompanying drawings is a plan view of the optical card 6. The optical card 6 comprises a substrate formed of plastics or the like, and a recording layer 20 of silver salt, dyestuff or the like formed on the substrate. On the recording layer 20, there are preformed a plurality of parallel tracking tracks 21 optically detectable by their unevenness or the difference in reflectance with respect to the adjacent portions of the card. These tracking tracks 21 are disposed at equal intervals, and a recording area in which information may be recorded is provided between the adjacent tracking tracks.
FIG. 4 of the accompanying drawings is an enlarged view of the recording surface of the optical card 6 illustrating the recording process using the apparatus of FIG. 1. There are recording areas 22.sub.1 and 22.sub.2 between the tracking tracks 21.sub.1, 21.sub.2 and 21.sub.3 The beam spots S.sub.1 and S.sub.3 are applied so as to partly overlap the tracking tracks 21.sub.1 and 21.sub.2, and the beam spot S.sub.2 is applied to the recording area 22.sub.1. If the applied positions of the beam spots S.sub.1 -S.sub.3 deviate in a direction perpendicular to the tracks, there occurs an imbalance between the quantity of light of the reflected beam from the spot S.sub.1 and the quantity of light of the reflected beam from the spot S.sub.3. Accordingly, by differentiating a signal obtained by detecting the lights from these spots as shown in FIG. 1, there is obtained a tracking signal S.sub.T indicative of the amount and direction of the deviation. By moving the objective lens on the basis of this tracking signal, the beam spot S.sub.2 is accurately directed to the recording area between the tracking tracks and information can be recorded as indicated by record pits 23. When information is to be recorded in the recording area 22.sub.2, the beam spots S.sub.1, S.sub.2 and S.sub.3 may be applied to the tracking track 21.sub.2, the recording area 22.sub.2 tracking track 21.sub.3, respectively. The light intensity distributions of the beam spots S.sub.1 and S.sub.3 on the optical card are usually Gaussian distributions which are high in the central portion and therefore, if these beam spots are rendered so that one half of each of the beam spots overlaps the tracking track, the variation in quantity of light by track deviation is great and tracking signal detection of high sensitivity can be accomplished.
Now, in the method of detecting the tracking signal as described above, when there is a defect in the tracking tracks, there has been the undesirable possibility that the beam spot S.sub.2 for recording and reproduction deviates from the recording area 22.sub.1.
Such a situation is shown in FIG. 5 of the accompanying drawings. FIG. 5 shows a situation in which a defective portion 25 is in the tracking track 21.sub.1 shown in FIG. 4 and FIG. 5 further shows times t.sub.0 -t.sub.1 corresponding to the scanning position of the beam spot S.sub.1 applied to this track.
The reflectance of the defective portion 25 is lower than that of the normal or unaffected portions of tracking tracks 21.sub.1 and 21.sub.2.
At the time t.sub.0, the spot S.sub.1 overlaps a normal or unaffected portion of the track 21.sub.1. Next, when at the time t.sub.1, the spot S overlaps the defective portion 25, there arises an imbalance between the quantity of light of the reflected beam from the spot S.sub.1 and the quantity of light of the reflected beam from the spot S.sub.3, whereby a fluctuation occurs in the applied position of the spots to effect an auto tracking operation. However, if as shown, the defective portion 25 is relatively wide, at time t.sub.1 ', the spot S.sub.2 will deviate from the recording area 22.sub.1 and normal recording and reproduction of information will become impossible.
Also, the situation in which an abnormality occurs in the control operation due to such defect or the like of the medium as described above might also affect the focusing control.